As a design rule becomes finer in a semiconductor integrated circuit, there is a concern over a reverse narrow channel effect and an increase of a leak current between elements of a MOSFET as a semiconductor device to be formed inside the semiconductor integrated circuit. As a remedy for these, ions of the same conductivity type as that of a semiconductor substrate are implanted into, for example, a side surface and a bottom surface of an element isolating region. This can reduce influence of the reverse narrow channel effect and the leak current between elements.
However, when a depletion-type n-channel MOSFET is formed, for example, there is a large difference in concentration due to an n-type impurity in the n-type channel region formed in a p-type semiconductor substrate and a p-type impurity formed in the side surface of the element isolating region. This might lower a breakdown voltage in junction between source/drain regions and the semiconductor substrate (or P-well region).
Hence, there is a desire of enhancing the breakdown voltage in a structure having the depletion-type channel region.